Lithofacies Distribution of the Mississippian Alida-Kisbey-Frobisher Stratigraphic ... · 2013. 5....

Lithofacies Distribution of the Mississippian Alida-Kisbey-Frobisher Stratigraphic Interval, Southeastern Saskatchewan

Congwei Ji 1 and Osman Salad Hersi 1

Ji, C. and Salad Hersi, O. (2013): Lithofacies distribution of the Mississippian Alida-Kisbey-Frobisher stratigraphic interval, southeastern Saskatchewan; in Summary of Investigations 2013, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of the Economy, Misc. Rep. 2013-4.1, Paper A-6, 15p.

Abstract The Alida-Kisbey-Frobisher strata form the upper part of the Mississippian Mission Canyon Formation in southeastern Saskatchewan. These strata host a number of prolific oil and gas pools in Saskatchewan, as well as in neighbouring North Dakota and Montana. The Alida-Kisbey-Frobisher interval consists of carbonate, siliciclastic and evaporite interbeds. Preliminary core, well log and petrographic studies of these strata have allowed identification of lithologic attributes and lithostratigraphic architecture of the studied rock succession. Seven lithofacies have been identified: 1) oolitic, bioclastic, oncolitic, peloidal, and intraclastic packstone to grainstone/rudstone (Facies PG); 2) oolitic, bioclastic, oncolitic, peloidal, and intraclastic mudstone to wackestone (Facies MW); 3) dolomudstone (Facies DS); 4) siltstone to sandstone (Facies SS); 5) sandy dolomudstone (Facies SD); 6) sandy packstone to grainstone (Facies SP); and 7) anhydrite (Facies AH). The overall depositional setting is tentatively interpreted as a peritidal environment characterized by carbonate subtidal sand shoals with a landward lagoonal to tidal mud-flat system.

Keywords: lithofacies, mixed clastic–carbonate sediments, evaporites, Alida Beds, Kisbey interval, Frobisher Beds, Mississippian, Mission Canyon Formation, Madison Group, southeastern Saskatchewan, petrography.

1. Introduction The Mississippian stratigraphic succession of southeastern Saskatchewan is dominated by carbonate rocks with subordinate siliciclastic and evaporitic intervals. These strata accumulated on the northern flanks of the Williston Basin, a subcircular basin with a depocentre in North Dakota (Figure 1). Mississippian strata subcrop at the sub-Mesozoic unconformity producing northwest-southeast subcrop trends, along which many of the Mississippian hydrocarbon reservoirs occur in southern Saskatchewan, North Dakota and Montana (Fuzesy, 1960; Kent, 1987; Kent et al., 1988; Nickel and Yang, 2011). Many of these reservoirs are hosted in the Alida-Kisbey-Frobisher stratigraphic interval (Figure 2). This paper conveys the preliminary results of an ongoing Master of Science research project by the senior author, the focus of which is to examine the lithologic attributes within the Alida-Kisbey-Frobisher succession in southeastern Saskatchewan. To date, cores, geophysical logs, drill cuttings and thin sections have been studied (Figure 1, Table 1).

2. Geological Framework The present-day structural Williston Basin has an area of approximately 250 000 km2, with its centre located near the town of Williston, North Dakota (Kent and Christopher, 1994). Evolution of the basin has a long history of basement reactivation, and sedimentation on its northern margin (southern Saskatchewan) beginning in the Cambrian period (Kent and Christopher, 1994). The basin occupied a large area of the western North American craton during the Mississippian (Bjorlie and Anderson, 1978; Lake, 1991). During the late Devonian to early Mississippian, the major tectonic elements were the Antler Orogenic Belt, the Antler Foreland Basin, the Prophet Trough, and the Central Montana Trough (Figure 1). Uplift of the Sweetgrass Arch during this time interval caused an isolation of the Williston Basin and its disconnection from the large Western Canada Sedimentary Basin. However, the basin was connected to the Pacific Ocean via the Montana Trough (Gerhard et al., 1991; Reid and Dorobek, 1993) (Figure 1). Mississippian sediments were deposited in a tectonically stable environment (Richards, 1989; Rott and Qing, 2005) and their lateral and vertical (temporal) distribution suggests that they evolved as a transgressive-regressive cycle equivalent to the upper part of Sloss’s (1963) Kaskaskia Supersequence. Mississippian-age strata are dominated by carbonates and succeed an environment previously dominated by marine mud and sand deposits: that is, latest Devonian – earliest Mississippian shales and sandstones of the Bakken Formation (Figure 2) (Halabura et al., 2007; Angulo and Buatois, 2011; Imam et al., 2011). Due to a lull in orogenic movement in the Early Mississippian, the Williston Basin became relatively stable, with reduced siliciclastic input and the establishment of a carbonate

1 Department of Geology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2.

Saskatchewan Geological Survey 1 Summary of Investigations 2013, Volume 1

Figure 1 – Left: Location map showing the study area in southeastern Saskatchewan, configuration of the Williston Basin, and the location of the Sweetgrass Arch, Montana Trough and Antler Foreland Basin (modified from Nimegeers, 2006). Right: Map showing the distribution of wells examined in the study area, and the line of cross section illustrated in Figure 3.


platform, represented by the Lodgepole Formation/Souris Valley Beds (Figure 2) (LeFever et al., 1991). Carbonate accumulation persisted throughout the deposition of the Madison Group. However, evaporite and sandstone intercalations in the upper part of the Madison Group (Nimegeers, 2006) indicate intermittent sea level fluctuations, restriction of the depositional environment, and episodic siliciclastic progradations during deposition of the Mississippian Madison Group.

Figure 2 – Stratigraphic chart of Mississippian strata in southeastern Saskatchewan (modified from Saskatchewan Ministry of the Economy, 2011).


Table 1 – List of studied wells in southeastern Saskatchewan, of which 23 contain cores (1 to 23) and 4 have only drill cuttings (24 to 27). Well numbers are keyed to Figure 1.

Well No. Well ID Licence Number

Cored Interval (m)

Cutting Interval Checked

(m)

Numbers of Thin

Sections

1 01/04-09-008-06W2 67B002 1228.0-1243.6

2 11/13-13-008-06W2 87I040 1181.0-1199.0 17

3 01/03-08-001-06W2 57D025 1759.8-1891.0

4 21/03-32-007-05W2 84A107 1224.0-1241.4

5 01/03-26-004-05W2 55J092 1374.0-1483.2 1

6 41/03-24-007-04W2 93H085 1208.0-1226.8 2

7 41/04-22-007-03W2 08J439 1195.0-1213.3

8 01/06-01-006-03W2 62L012 1220.7-1246.8

9 01/07-34-005-02W2 85G245 1227.0-1245.0

10 01/09-23-002-02W2 56J006 1371.0-1440.1 1475.2-1490.2

11 01/05-20-006-01W2 00F231 1190.0-1208.2

12 01/02-27-005-01W2 85K115 1183.0-1202.3

13 41/10-09-004-01W2 08C362 1238.5-1256.8

14 41/09-33-005-34W1 87K208 1152.0-1154.8 1155.0-1173.0

15 31/06-02-004-33W1 97G141 1173.5-1192.8

16 01/04-12-004-33W1 96I248 1164.6-1182.6 1183.0-1201.0

33

17 11/06-12-004-33W1 96K245 1164.0-1199.0 4

18 01/16-10-005-32W1 55C028 1074.4-1116.5 7

19 01/08-01-004-32W1 67F004 1091.2-1106.5

20 21/16-05-003-32W1 98F112 1192.8-1211.2

21 11/15-18-001-32W1 90G125 1281.5-1300.5 8

22 02/16-27-002-31W1 56K120 1100.5-1127.2

23 50/11-19-002-30W1 72L015 1075.0-1092.8

24 41/13-16-003-05W2 08G476 1595-1615

25 41/15-16-002-04W2 08J206 1602-1620

26 91/16-32-001-05W2 08H550 1720-1735

27 41/09-32-001-03W2 08C328 1623-1635


3. Stratigraphy Post-Bakken Mississippian rocks in Saskatchewan have been subdivided into two groups: the Madison Group, and the overlying, erosionally reduced Big Snowy Group (Figure 2). The former forms the bulk of the Mississippian succession in the province and consists, in ascending order, of the Lodgepole, Mission Canyon and Charles formations (Figure 2). The Mission Canyon Formation of southeastern and south-central Saskatchewan consists of four stratigraphic units, in ascending stratigraphic order: Tilston Beds, Alida Beds, Kisbey interval, and Frobisher Beds (Marsh, 2006; Nickel and Yang, 2011; Figure 2). The presence of clastic layers within the Mississippian strata of southeastern and south-central Saskatchewan, as well as in Montana and North Dakota, played a significant role in influencing the stratigraphic subdivision of the Mission Canyon Formation. Thomas (1954) first recognized a discrete sandstone/siltstone interval in the Mission Canyon Formation of southeastern Saskatchewan and named it MC4. This sandstone interval was later called the Kisbey Sandstone by Fuller (1956). In southeast Saskatchewan two sandstone intervals were recognized by Edie (1958) and Fuzesy (1960). Legault (1999) adopted the terms K1, K2 and K3 (see Figure 2) for marker beds in his study area, following marker-bed terminology used within the Mission Canyon Formation of North Dakota (e.g., Harris et al., 1966; Carlson and LeFever, 1987). Halabura (2006) took a sequence-stratigraphic approach, adopting an allostratigraphic subdivision of the Mission Canyon Formation (e.g., Alida Alloformation, Kisbey Alloformation, etc.). Kent (2007, p.3) employed a different stratigraphic scheme, where he lumped “all beds containing quartz sand or silt” under the name “Kisbey interval”.

In this paper, we follow the nomenclature established by Kent (2007). The Alida–Kisbey boundary is put at the base of the first appearance of sandstone or sandy/silty carbonate beds and the top of Kisbey is put at the top of the youngest (highest) bed of sandstone or sandy/silty carbonate (Figures 2 and 3). These lower and upper beds in the Kisbey interval are commonly characterized by a relative higher gamma ray response and lower photoelectric curve than the underlying Alida Beds and the overlying Frobisher Beds. In areas where no quartz grains are present, the Kisbey interval is composed predominantly of argillaceous dolomudstone, which is characterized by a higher gamma ray response.

The interpretation of the origin and depositional setting of the siliciclastic and carbonate strata of the Kisbey interval of southeast Saskatchewan has been discussed in different papers (Howard, 2000; Kent, 2004, 2007; Halabura and Costa, 2005; Halabura, 2006; Ji and Salad Hersi, 2013). Kent (2004, 2007) endorsed a mixed eolian–shallow marine depositional setting while Halabura (2006) envisaged a fluviomarine system for the mixed carbonate, clastic, and evaporite succession of the Mission Canyon Formation. Within the Kisbey sandstone, Howard (2000) recognized three sedimentary environments: wave-influenced sandstone shoals, storm-transported sand deposits in an open marine environment, and tidal channel sands. Many researchers have indicated that the Mission Canyon Formation deposits are part of an overall shallowing-upward trend in the Mississippian (e.g., Kent, 1984; Waters and Sando, 1987) and that cyclicity can be identified on several scales (e.g., Elliot, 1982; Hoff, 1987; Lake, 1991).

4. Lithologic Attributes of Alida, Kisbey, and Frobisher Strata

a) General Description The Alida Beds consist of limestone, and minor dolostone and anhydrite interbeds (Fuzesy, 1960; Rott and Qing, 2005) and are overlain by the Kisbey interval, which consists of interbeds of limestone, dolostone, siltstone/sandstone, and sandy carbonates (Kent, 2007). The Frobisher Beds are dominated by limestone and dolostone lithofacies (Fuzesy, 1960; Marsh, 2006) and succeed the uppermost sandstone or sandy carbonate bed of the Kisbey interval. The top of the Mississippian succession in southeastern Saskatchewan is marked by an unconformity (Figures 2 and 3).

b) Lithofacies Properties The various lithofacies identified in the Alida, Kisbey and Frobisher succession in the study area are based on the study of 23 cored wells and cuttings descriptions from 4 more (Figure 1), wireline logs, and 72 thin sections from 7 of the cored wells (Table 1). Core descriptions and petrographic analysis of the studied strata allow recognition of seven lithofacies, which comprise: 1) oolitic, bioclastic packstone to grainstone/rudstone (Facies PG); 2) oolitic, bioclastic mudstone to wackestone (Facies MW); 3) dolomudstone (Facies DS); 4) siltstone to sandstone (Facies SS); 5) sandy dolomudstone (Facies SD); 6) sandy packstone to grainstone (Facies SP); and 7) anhydrite (Facies AH). The characteristics of these seven lithofacies are described in Table 2.


Figure 3 – Cross section showing the lithologies of Alida Beds, Kisbey interval and Frobisher Beds in cores from five wells in the study area; the locations of these wells are shown in Figure 1. Datum is the sub-Mesozoic unconformity surface, the depth of which was determined on wireline logs for the three wells in which this contact is not cored. Dashed lines indicate the top of the Alida as inferred from wireline logs.


Table 2 – Lithofacies descriptions of the studied strata, and their inferred depositional environments.

Facies Lithofacies Description Sedimentary Structures Distribution Depositional Environment

PG Packstone/ grainstone/ rudstone

Light to dark grey packstone, grainstone

and rudstone

Fenestral structure and minor microbial laminations

Mainly in Alida and Frobisher

beds and locally in Kisbey interval

Subtidal

MW Mudstone/ wackestone

Light to dark grey mudstone/wackstone

Fenestral structure, microbial laminations

Distributed throughout the Alida-Kisbey-

Frobisher interval

Lagoon to intertidal

DS Dolomudstone Pale grey to grey dolomudstone

Fenestral structure, mudcracks and microbial

laminations (including domal stromatolites)

Distributed throughout the Alida-Kisbey-

Frobisher interval

Supratidal to upper intratidal

SS Sandstone/ siltstone

Yellowish grey to dark grey sandstone/siltstone

Massive, bioturbated, tabular and trough cross-laminations and horizontal laminations

Kisbey interval Tidal channel

SD Sandy dolomudstone

Pale to dark grey sandy dolomudstone Massive Kisbey interval Supratidal to

upper intratidal

SP Sandy packstone/ grainstone

Grey sandy packstone/ grainstone Fenestral structure Kisbey interval Subtidal

AH Anhydrite White and red anhydrite Massive and nodular Frobisher and Alida beds Supratidal

Packstone to Grainstone/Rudstone Lithofacies (PG) Description: The packstone to grainstone/rudstone facies is characterized by thick- to medium-bedded, light to dark grey limestone. Sedimentary structures in this lithofacies include a fenestral fabric and minor microbial laminations that drape the grainstone layers (Figure 4A); stylolites are common. The PG lithofacies has fair to good porosity (e.g., Figure 4A) and is commonly oil stained. Anhydrite, dolomite or calcite cements locally infill the pore space (Figure 4C), and thus reduce the overall porosity of the unit. Although this lithofacies occurs throughout the Alida, Kisbey and Frobisher units, it occurs most commonly within the upper part of the Alida and Frobisher beds. Contacts of this facies with Kisbey sandstone units are gradational to sharp.

Microscope analysis reveals that ooids and bioclasts are the most common framework grains (Figures 4B, 4C) of this lithofacies. Granule- to pebble-size oncoids are locally abundant, forming oncolitic rudstone (Figure 4D) with subordinate peloids, oncoids and intraclasts, although the peloids and intraclasts may have originated as micritized oncoids, ooids or bioclasts. The bioclasts include crinoids, brachiopods, bivalves, foraminiferas, gastropods, bryozoans, and, less commonly, calcareous green algae. The packstone/grainstone lithofacies typically consists of 80 to 90% ooids and bioclasts. Ooids commonly show a poorly preserved internal (cortex) structure due to extensive micritization and, thus, many grains that appear to be peloids may be pervasively micritized ooid grains. This facies is locally dolomitized. The cements of this lithofacies are dominated by anhydrite, dolomite and calcite.

Interpretation: The lithofacies PG accumulated in a high-energy, normal marine depositional environment (Edie, 1958; Kent, 1984; Perras, 1990; Rott and Qing, 2005), representing a carbonate sand shoal. The fact that no exposure surfaces are recognized may suggest that the lithofacies developed in a shallow subtidal setting.


Figure 4 – A) Core photograph of oolitic grainstone showing microbial lamination (Mb), birdseye structure (Be), aggregate grains (Ag), oolite grains (Oo), and anhydrite cement (An): Frobisher Beds, 1093.5 m, 01/08-01-004-32W1; 67F004 (well 19 on Figure 1). B) Photomicrograph of oolitic grainstone showing oolite grains (Oo), dolomite cement (Do), and pore space (Po, porosity is filled by blue epoxy). The thin section is stained with Alizarin Red. Photo is taken under plane polarized light: Frobisher Beds, 1286.3 m, 11/15-18-001-32W1; 90G125 (well 21 on Figure 1). C) Photomicrograph of oolitic bioclastic grainstone showing oolite grains (Oo), bioclasts (Bc), calcite cement (Cc), and pore space (Po). Photo is taken under cross-polarized light: Kisbey interval, 1195.0 m, 11/13-13-008-06W2; 87I040 (well 2 on Figure 1). D) Core photograph of oncolitic rudstone showing microbial lamination (Mb), oncoids (On), anhydrite cement (An), and pore space (Po): Frobisher Beds, 1154.0 m, 41/09-33-005-34W1; 87K208 (well 14 on Figure 1).

Mudstone to Wackestone Lithofacies (MW) Description: The mudstone to wackestone lithofacies consists of laminated to thickly bedded, light to dark grey limestone. Sedimentary structures include microbial laminations and fenestral (birdseye) structure. This lithofacies has 5 to 10% vuggy porosity (Figures 5A, 5B) and subordinate intercrystalline porosity. The lithofacies is typically oil stained. Some of the pores are filled by anhydrite and calcite cements. This lithofacies is distributed throughout the Alida, Kisbey and Frobisher intervals.

Framework grains of Facies MW are dominated by bioclasts, ooids, intraclasts, and subordinate oncoids and peloids (Figure 5B shows a peloidal, intraclastic wackestone). Oncoids have irregular lamination, a large grain size and commonly appear to be floating within the micrite matrix. Most oncoid grains are simple, but in many cases, several oncoids bind together, forming a large composite oncoid. The matrix of Facies MW is typically composed of anhedral micrite, partly replaced by very fine crystalline dolomite rhombs. The cement is predominately anhydrite and dolomite.

Interpretation: The textural attributes (high mud content) and sedimentary structures of this lithofacies indicate a low-energy depositional environment, in which accumulation took place in a protected lagoonal to intertidal mudflat setting (Howard, 2000; Rott and Qing, 2005; Pratt, 2010). The depositional site was probably protected by the carbonate sand shoals of lithofacies PG.


Figure 5 – A) Core photograph of mudstone with good dissolution porosity (Po): Frobisher Beds, 1185.5 m, 01/02-27-005-01W2; 85K115 (well 12 on Figure 1). B) Photomicrograph of peloidal intraclastic wackestone with intraclasts (In), peloids (Pe), and pore space (Po); the thin section is stained with Alizarin Red. Photo is taken under cross-polarized light: Alida Beds, 1116.1 m, 01/16-10-005-32W1; 55C028 (well 18 on Figure 1).

Dolomudstone Lithofacies (DS) Description: This lithofacies consists of laminated to thickly bedded, pale grey to grey dolomudstone. Fenestral (birdseye) fabric commonly filled by anhydrite, mudcracks and microbial laminations (including domal stromatolites) are present (Figures 6A, 6B). This lithofacies appears dense, with no significant pore space. In most cases, the contact with the underlying sandstone lithofacies is gradational. Lithofacies DS is present throughout the Alida-Kisbey-Frobisher interval.

Petrographic examination of this lithofacies shows a dolomicrite texture (<4 microns) with a minor amount of fine (~100 microns) dolomite crystals. Dolomicrites are mainly anhedral, sometimes have relict bioclasts and other possibly peloidal grains. Fine crystalline dolomite crystals appear as euhedral rhombs, which usually float in the dolomicrite. Overall, the dolomudstone lithofacies contains less than 5% anhydrite and dolomite cements.

Interpretation: The dolomudstone texture, relative lack of sedimentary structures, and the association with the anhydrite lithofacies suggest deposition within a restricted environment (Kent, 1984). The dolomudstone lithofacies is envisaged as having accumulated in a supratidal to upper intertidal mud-flat setting (Shinn, 1983).

Figure 6 – A) Dolomudstone lithofacies showing birdseye structure (Be): Frobisher Beds, 1157.1 m, 41/09-33-005-34W1; 87K208 (well 14 on Figure 1). B) Domal stromatolite (St) of fine crystalline dolomudstone: Frobisher Beds, 1221.4 m, 01/06-01-006-03W2; 62L012 (well 8 on Figure 1).


Sandstone/Siltstone Lithofacies (SS) Description: This lithofacies is a thinly to thickly bedded, yellowish grey to dark grey siltstone/sandstone. Most of the beds are massive with no internal structures. The unit is commonly interbedded with other carbonate lithofacies, but the continuous section of the sandstone lithofacies may locally reach as thick as 20 m (e.g., 01/04-12-004-33W1; 96I248, well 16 on Figure 1). Some beds do, however, have sedimentary structures—including tabular and trough cross-laminations, and horizontal laminations—and may be sparsely to moderately bioturbated (probably Skolithos and other unidentifiable burrows; Figure 7A). This facies has moderate porosity and is oil stained in some cases. The sandstone/siltstone is in contact with a variety of carbonate lithofacies. Lower contacts are commonly sharp, whereas the upper contacts appear gradational into the carbonate lithofacies. The contact surfaces between the sandstone/siltstone beds are sharp to erosional. The sandstone/siltstone lithofacies is confined to within the Kisbey interval and is characterized by lateral discontinuity and, thus, it is difficult to correlate from one well to another.

In thin sections, the framework grains are dominated by quartz (>80%), followed by feldspar (~15%) and minor chert grains (<5%) (Figure 7B). This facies is generally matrix-free but locally shows a dolomudstone matrix similar to the DS facies. Individual grains of sandstone/siltstone lithofacies are predominantly fine grained, subangular, and well sorted. Rare carbonate grains, such as ooids, are locally present within this lithofacies. The cements of this facies are anhydrite, calcite and dolomite.

Interpretation: The sandstone/siltstone lithofacies has very few sedimentary structures, rendering interpretation difficult. The presence of sharp and erosional contacts, cross laminations and vertical burrows suggests a moderate- to high-energy depositional environment (Campbell, 1967; Allen, 1982). The fact that the unit has a gradational upper contact with the overlying carbonates and is laterally discontinuous (not unequivocally traceable from one well to the next) suggests that it possibly accumulated in intertidal channels that cut through the shallow-marine carbonate platform. It is envisaged that the siliciclastic sediments in the tidal channels were fed by intermittent fluvial influx from the northeast of the Williston Basin, particularly during relative sea level drop. Further refinement of the depositional system for this lithofacies, and its sedimentary fill architecture is in progress.

Figure 7 – Kisbey interval, 01/04-12-004-33W1; 96I248 (well 16 on Figure 1). A) Core photograph of the sandstone lithofacies (SS). The unit contains vertical burrows (blue outline, possibly Skolithos). The photo is taken at 1194.5 m. B) Photomicrograph of the SS facies showing fine- to medium-grained sandstone dominated by quartz (Qa) and chert (Ch) grains with fine crystalline dolomudstone matrix (Do). The rock is partially cemented by anhydrite (An). Photo is taken under cross-polarized light: 1194.3 m.

Sandy Dolomudstone Lithofacies (SD) Description: This lithofacies is dominated by massive, pale to dark grey sandy dolomudstone that varies in thickness. Most of the beds have no sedimentary structures. This lithofacies is dense with no significant pore space. In most cases, the lower contact is gradational while the upper contact is sharp or erosional. Lithofacies SD is distributed within the Kisbey interval. It is interbedded with the dolomudstone, sandstone and limestone lithofacies.

Petrographic examination shows that the sandy dolomudstone lithofacies is composed of dolomicrite, with a minor amount of fine quartz grains (Figure 8). Dolomicrite crystals are anhedral. Quartz grains are fine and subangular, and similar to the grains within the sandstone lithofacies. The cement of this lithofacies consists of anhydrite and dolomite.


Interpretation: The sandy nature of the dolomudstone lithofacies, its microcrystalline texture and relationship with the sandstone and dolomudstone lithofacies suggests a supratidal to upper intertidal mud flat depositional setting (Pratt, 2010).

Figure 8 – Kisbey interval; 01/04-12-004-33W1;96I248 (well 16 on Figure 1). A) Photomicrograph of the sandy dolomudstone lithofacies with fine quartz grains (Qa), dolomicrite (Do), and chert (Ch). Photo is taken under cross-polarized light: 1183.4 m. B) Photomicrograph of sandy dolomudstone with fine quartz grains (Qa) and dolomicrite (Do). Photo is taken under plane polarized light: 1196.6 m.

Sandy Packstone/Grainstone Lithofacies (SP) Description: The packstone/grainstone facies is characterized by grey sandy packstone/grainstone units that vary in thickness. Sedimentary structures include fenestral (birdseye) structure. This facies occurs in the Kisbey interval and is associated with the Kisbey sandstone lithofacies (SS), with which it is in gradational to sharp contact.

Microscopic examination reveals that the most common framework grains are fine quartz, ooids, and bioclasts (Figure 9). The ooids and bioclasts are partially micritized. The bioclasts include crinoids, brachiopods, bivalves, foraminiferas, gastropods, bryozoans, and green algae. The sandy packstone/grainstone lithofacies typically consists of 15% fine quartz and 65% carbonate framework grains (ooids and bioclasts, Figure 9). The cements of this lithofacies are anhydrite, dolomite, and some calcite.

Interpretation: The lithofacies SP is a sandy version of the packstone to grainstone/rudstone (PG) lithofacies. It is interpreted to represent a high-energy, normal marine subtidal depositional environment similar to that of the PG facies but with some clastic input from sand-transporting channels that deposited the sandstone/siltstone (SS) lithofacies.

Figure 9 – Kisbey interval; 11/13-13-008-06W2; 87I040 (well 2 on Figure 1). A) Photomicrograph of sandy oolitic grainstone with quartz grains (Qa), ooids (Oo), calcite cement (Cc), and pore spaces (Po). Photo is taken under cross-polarized light: 1198.8 m. B) Photomicrograph of sandy bioclastic grainstone with quartz grains (Qa), bioclasts (Bc), ooids (Oo), and calcite cement (Cc). Photo is taken under cross-polarized light: 1197.1 m.


Anhydrite Lithofacies (AH) Description: This lithofacies is present in Frobisher and Alida beds. In the Frobisher Beds, it consists of laminated to medium-bedded, reddish to whitish coloured anhydrite that does not show any internal structures except subordinate zones that show nodular texture. It is locally interbedded with the dolomudstone (DS) lithofacies (Figure 10A). In the Alida Beds, lithofacies AH has been recognized from one well (01/16-10-005-32W1; 55C028, well 18 on Figure 1) and is characterized by massive, white-coloured anhydrite (Figure 10B). Like the anhydrite layers of the Frobisher Beds, the anhydrite facies of the Alida Beds is also associated with the fine crystalline dolostone lithofacies (DS).

Interpretation: Anhydrite indicates deposition within an evaporitic environment (Kent, 1984; Waters and Sando, 1987). The anhydrite lithofacies is interpreted as a supratidal (sabkha) accumulation.

Figure 10 – A) Interlayered dolomudstone (Do) and anhydrite (An): Frobisher Beds, 1075.5 m, 50/11-19-002-30W1; 72L015 (well 23 on Figure 1). B) Massive anhydrite: Gainsborough Evaporite, Alida Beds, 1085.1 m, 01/16-10-005-32W1; 55C028 (well 18 on Figure 1).

5. Conclusions Lithologic analysis of the Alida-Kisbey-Frobisher succession allowed recognition of seven lithofacies units, which comprise: 1) packstone to grainstone/rudstone (Facies PG); 2) mudstone to wackestone (Facies MW); 3) dolomudstone (Facies DS); 4) siltstone to sandstone (Facies SS); 5) sandy dolomudstone (Facies SD); 6) sandy packstone/grainstone (Facies SP); and 7) anhydrite (Facies AH). The sandstone and sandy carbonate units are confined to within the Kisbey interval, but the cleaner carbonate units are present in different layers of the Alida to Frobisher intervals. The overall depositional setting can be summarized as a peritidal environment characterized by carbonate subtidal sand shoals with a landward lagoonal to tidal mud-flat system. The sandstone lithofacies of the Kisbey interval suggests a connection between tidal creeks that cut a carbonate-dominated shallow platform and a fluvial system that brought the clastic particles into the basin. Details of the architectural relationship among the different carbonate and clastic lithofacies and their significance in terms of the depositional system of the studied rock interval are in progress.

6. Acknowledgements This project is financially supported by the Government of Saskatchewan through the Saskatchewan Geological Survey. The authors extend their gratitude to the staff of the Saskatchewan Ministry of Economy’s Subsurface Geological Laboratory for providing free access to core, as well as providing us with pertinent data and materials. We also thank the staff of the core facility for their core handling and support. This paper has benefited from reviews by Fran Haidl, Dr. G. Chi and Dr. S. Bend.

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